The present invention relates generally to the field of digital communications. More specifically, the present invention relates to multipoint TDM data distribution systems in which different modulation formats are applied to different portions of a TDM transmission signal.
Time domain multiplexing (TDM) of a point-to-multipoint communication signal broadcast from a single transmission point to a plurality of reception points potentially provides numerous benefits. Using TDM, a single frequency channel operating in a given frequency band is subdivided to form two or more concurrent temporal channels which occupy the single frequency channel at discrete time intervals. One benefit of TDM is that each of the various reception points may be assigned its own private temporal channel. Another of the benefits of TDM is that the single frequency channel is continuously available at all reception points for maintaining receiver synchronization, even during temporal channels not assigned at a given reception point. Thus, when the assigned intervals occur for various temporal channels, little time is wasted in achieving synchronization. Another benefit is that the use of a continuous single frequency channel potentially permits the use of long coding blocks, which are advantageous for maximizing coding gain. Coding gain refers to the portion of gain shown by a communication link from forward error correction (FEC) achieved by encoding to-be-transmitted payload data. The payload data are encoded in accordance with a predetermined mathematical algorithm which combines error control bits with the payload data. The error control bits encoded with the payload data are used by a receiver to detect and correct errors.
On the other hand, the potential benefits of TDM have been difficult to achieve since undesirable sacrifices have conventionally been required. For example, it is desirable for a communication system to communicate using a wide variety of modulation formats and coding rates and for a media access controller (MAC) to have as much flexibility as possible in specifying such modulation formats and coding rates along with the durations of temporal channel intervals. Thus, when and where an excellent reception capability exists, such as on a clear day with no obstructions between a transmission point and a nearby reception point, a high modulation order and low coding rate may be specified to convey a greater amount of payload data in less time using a given power level and spectral occupancy. However, for other reception points that might be further away or partially obstructed from the transmission point, lower modulation orders and/or higher coding rates may be needed to successfully convey data at the given power level and spectral occupancy. The use of such lower modulation orders and/or higher coding rates causes less payload data to be conveyed in a given amount of time.
To efficiently use the assigned spectrum, a MAC is desirably empowered to offer each reception point the highest modulation order and lowest coding rate that will achieve a desired bit error rate (BER) using a given amount of power while confining the frequency channel to a given spectral occupancy. Conventional TDM communication systems have not been successful in achieving the potential benefits of TDM, while concurrently transmitting different temporal channels at different modulation orders and/or coding rates, and giving a MAC flexibility in assigning modulation order, coding rate, and temporal channel interval so as to utilize spectrum as efficiently as possible.
One reason conventional TDM systems have failed to provide flexibility in modulation order and coding rate concerns initiating and closing the intervals which define temporal channels. In order to maximize coding gain, higher performance digital communication systems conventionally employ more than one encoder in the transmitter and more than one decoder in the receiver. Such an architecture may be referred to as concatenated encoding. This desirable encoding architecture may use an inner convolutional encoding/decoding algorithm paired with an outer block or Reed Solomon encoding/decoding algorithm, or two separate convolutional encoding/decoding algorithms (i.e., turbo encoding).
Convolutional decoders tend to make errors in bunches. If a convolutional decoder fails to correct an error, it is likely to output several errors in a brief interval. The decoder paired with a convolutional decoder is much more successful at detecting and correcting errors if it does not see bursts of errors, but sees the errors spread out in time. Consequently, interleaving in the transmitter and deinterleaving in the receiver are conventionally performed between the two coding algorithms to temporally spread adjacent bits over a large period of time so that a bunch of errors appearing at the output of a convolutional decoder are less likely to be presented together at the paired decoder. While this technique improves coding gain performance, it also makes the initiation and closing of temporal channels which convey interleaved data indistinct until conveyed data may be detected and deinterleaved. The complication of interleaved modulation formats and/or coding rates which would occur at the beginning and ending of discrete temporal channels using conventional techniques has prevented the provision of variable modulation formats and coding rates in TDM communication systems.
One solution to the problem of interleaved modulation formats at the initiation and closing of temporal channels might be to flush interleavers using dummy data at the temporal channel boundaries. That way, each modulation format would be intermingled only with dummy data, and a receiver need not simultaneously detect data modulated using different modulation formats. However, this solution wastes payload-conveying capacity by conveying dummy data rather than payload data. Moreover, since the amount of wastage increases as temporal channel intervals shrink, a MAC would be undesirable constrained to prevent temporal channel intervals from becoming too short and thereby worsen wasted payload-conveying capacity.
It is an advantage of the present invention that an improved multipoint TDM data distribution system is provided.
Another advantage of the present invention is that a TDM data distribution system is provided which accommodates variable modulation formats and coding rates.
Another advantage of the present invention is that a TDM data distribution system is provided with improved flexibility in devising temporal channels because coding gain considerations are largely decoupled from temporal channel interval considerations.
Another advantage of the present invention is that the additional complexity required to implement the present invention is largely confined to a hub in a point-to-multipoint communication system.
Another advantage of the present invention is that it allows a data distribution system to improve the efficiency with which an assigned amount of spectrum is used to convey data.
These and other advantages are realized in one form by an improved multipoint transmitter for use in a digital time-division multiplex (TDM) communication system which transmits payload data to a plurality of reception points. The multipoint transmitter includes a first encoding forward error correction (FEC) processor configured to encode a first portion of the payload data by adding first error control bits to generate first FEC-encoded data modulated in accordance with a first modulation format. A second encoding FEC processor is configured to encode a second portion of the payload data by adding second error control bits to generate second FEC-encoded data modulated in accordance with a second modulation format. A pulse shaper has an input coupled to the first and second encoding FEC processors. The pulse shaper is configured to merge and filter the first and second FEC-encoded data into a substantially continuous transmission signal stream.
The above and other advantages are further realized in another form by an improved multipoint receiver for use in a digital time-division multiplex (TDM) communication system in which payload data are transmitted to a plurality of reception points. The multipoint receiver includes a downconversion section configured to receive a substantially continuous analog RF transmission signal and to generate a substantially continuous received data stream. A symbol tracking loop is coupled to the downconversion section. The symbol tracking loop is configured to synchronize to the received data stream to detect a substantially continuous baud clock. A decoding forward error correction (FEC) processor is configured to receive the received data stream and to generate FEC-decoded data in synchronism with the baud clock. A controller is coupled to the FEC processor. The controller is configured to selectively allow and prevent synchronous operation of the decoding FEC processor with the substantially continuous baud clock.